Image encoding device, image decoding device and program

ABSTRACT

An image encoding device (1) includes the cross-component intra predictor (171a) generates a predicted chroma block through cross-component intra prediction in which a chroma block to be encoded is predicted by referring to, as the neighbouring decoded pixels adjacent to the chroma block, decoded luminance pixels and decoded chroma pixels, a candidate generator (181) configured to generate candidates for an orthogonal transform type to be applied to orthogonal transform processing on prediction residuals that represent errors between the predicted chroma block and the chroma block; and a transformer (121) configured to perform the orthogonal transform processing on the chroma prediction residuals by using an orthogonal transform type selected from among the candidates generated by the candidate generator (181). The candidate generator (181) generates the candidates for the orthogonal transform type, depending on whether or not positions of the neighbouring decoded pixels referred to in cross-component intra prediction are to only any one of a top, a bottom, a left, and a right of a chroma block.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.17/175,098 filed Feb. 12, 2021, which is a Continuation of InternationalApplication No. PCT/JP2019/031561 filed Aug. 9, 2019, which claimsbenefit of priority to Japanese Patent Application No. 2018-152988 filedAug. 15, 2018, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an image encoding device, an imagedecoding device and a program.

BACKGROUND ART

In image encoding techniques, intra prediction is widely used, whichutilizes spatial correlation in a frame. In intra prediction, aprediction image is generated by referring to neighbouring decodedpixels adjacent to a prediction-target block, and predicting pixelvalues in the prediction-target block.

An image encoding device calculates prediction residuals that representerrors between the prediction image and the block, generates transformcoefficients by performing orthogonal transform processing on theprediction residuals, quantizes and entropy-encodes the transformcoefficients, and outputs encoded data (a bit stream).

For example, Patent Literature 1 discloses an image encoding deviceusing the intra predictor being capable of improving encoding efficiencyof the intra prediction mode of the chroma signal, in an intra predictorthat determines the intra prediction mode for a chroma signal for eachblock of an image (see Patent Literature 1). Conventionally, DCT type 2(DCT-2) has widely been used for an orthogonal transform type applied tothe orthogonal transform processing.

In HEVC described in Non Patent Literature 1, it is stipulated that notDCT-2 but DST type 7 (DST-7) is applied to a block of a designated sizeor smaller to which intra prediction is applied. Unlike DCT-2, DST-7 isan orthogonal transformation with asymmetric impulse responses, and is atransformation generating smaller coefficients for end points on oneside and larger coefficients for end points on another side.

Since intra prediction predicts a block by referring to neighbouringdecoded pixels adjacent to the block, it is highly probable that in theblock, prediction accuracy is high in an area close to the neighbouringdecoded pixels, and prediction accuracy is low in an area far from theneighbouring decoded pixels. DST-7 is applied to prediction residualswith such characteristics, whereby energy of the prediction residualscan be more efficiently concentrated.

On the other hand, as a mode of intra prediction, cross-component intraprediction (CCIP) is known that predicts chroma pixel values by usingneighbouring decoded luminance (luma) pixels.

Specifically, in cross-component intra prediction, decoded luminancepixels and decoded chroma pixels adjacent to a top side and a left sideof a chroma block are referred to, a luminance-chroma linear model forthe block is calculated, and chroma pixel values in the block arepredicted by using the calculated linear model. Note thatcross-component intra prediction is also referred to as CCLM(Cross-Component Linear Model) prediction in some cases.

Cross-component intra prediction can predict a chroma signal with highaccuracy, by using a fact that a relationship between signaldistributions of neighbouring decoded luminance and chroma pixels to thetop side and the left side of a block can be approximated to arelationship between luminance and chroma signal distributions in theblock. However, since top and left neighbouring decoded pixels arealways used, a correct linear model cannot be calculated when arelationship between luminance and chroma signal distributions differsbetween the top side and the left side, and consequently, chromaprediction accuracy is lowered.

In view of such a problem, Non Patent Literature 2 describes CCIP_A modeand CCIP_L mode as new intra prediction modes for chroma. The CCIP_Amode is an intra prediction mode in which a linear model is calculatedby using only top neighbouring decoded luminance and chroma pixels,instead of using top and left neighbouring decoded luminance and chromapixels. The CCIP_L mode is an intra prediction mode in which a linearmodel is calculated by using only left neighbouring decoded luminanceand chroma pixels. In Non Patent Literature 3, similar intra predictionmodes are described and an intra prediction mode corresponding to theCCIP_A mode is referred to as an LM-top mode and an intra predictionmode corresponding to the CCIP_L mode is referred to as an LM-left mode.

Moreover, Non Patent Literature 2 describes that in orthogonal transformprocessing on prediction residuals of a chroma block to which the CCIPmode, the CCIP_A mode, or the CCIP_L mode is applied, DCT-2 is appliedin vertical and horizontal directions, or DCT-8 or DST-7 is selectivelyapplied in each of the vertical and horizontal directions.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.2018-078409 Non Patent Literature

Non Patent Literature 1: Recommendation ITU-T H.265, (12/2016), “Highefficiency video coding”, International Telecommunication Union

Non Patent Literature 2: JVET-J0025, “Description of SDR, HDR and 360°video coding technology proposal by Qualcomm and Technicolormediumcomplexity version”, retrieved on May 22, 2018, Internet <URL:http://phenix.it-sudparis.eu/jvet/doc_end_user/documents/10_San%20Diego/wg11/JVET-J0025-v4.zip>

Non Patent Literature 3: JVET-J0018, “Description of SDR video codingtechnology proposal by MediaTek” retrieved on May 22, 2018, <URL:http://phenix.it-sudparis.eu/jvet/doc_end_user/documents/10_San%20Diego/wg11/JVET-J0018-v2.zip>

DISCLOSURE OF INVENTION Technical Problem

However, it is highly probable that bias occurs in an energydistribution trend of prediction residuals of a chroma block to whichthe CCIP_A mode or the CCIP_L mode is applied.

For example, when the CCIP_A mode is applied, since top neighbouringdecoded luminance and chroma pixels are used to calculate a linearmodel, it is highly probable that prediction accuracy is high in anupper-end area of a block of interest. On the other hand, it is highlyprobable that prediction accuracy is low in a lower-end area that is farfrom the neighbouring decoded pixels used to calculate the linear model.When DCT-8 is applied in the vertical direction to prediction residualswith such characteristics, a problem arises that encoding efficiency islowered because energy of the prediction residuals is not concentrated.

Similarly, when the CCIP_L mode is applied, since left neighbouringdecoded luminance and chroma pixels are used to calculate a linearmodel, it is highly probable that prediction accuracy is high in aleft-end chroma area of a block of interest, but it is highly probablethat prediction accuracy is low in a right-end area that is far from theneighbouring decoded pixels used to calculate the linear model. WhenDCT-8 is applied in the horizontal direction to prediction residualswith such characteristics, a problem arises that encoding efficiency islowered because energy of the prediction residuals is not concentrated.

In view of the above, an object of the present invention is to provideimage encoding device, image decoding device and program in whichencoding efficiency is improved when cross-component intra prediction isused.

Solution to Problem

An image encoding device according to a first feature includes: across-component intra predictor configured to generate a predicted blockof a second component through cross-component intra prediction in whicha block of the second component to be encoded is predicted by referringto, as neighbouring decoded pixels adjacent to the block of the secondcomponent, decoded pixels of a first component and decoded pixels of thesecond component; a candidate generator configured to generatecandidates for an orthogonal transform type to be applied to orthogonaltransform processing on prediction residuals that represent errorsbetween the predicted block of the second component and the block of thesecond component; and a transformer configured to perform the orthogonaltransform processing on the prediction residuals by using an orthogonaltransform type selected from among the candidates generated by thecandidate generator, wherein the candidate generator is configured togenerate the candidates for the orthogonal transform type, based onpositions of the neighbouring decoded pixels referred to in thecross-component intra prediction.

An image decoding device according to a second feature includes: across-component intra predictor configured to generate a predicted blockof a second component through cross-component intra prediction in whicha block of the second component to be decoded is predicted by referringto, as neighbouring decoded pixels adjacent to the block of the secondcomponent, decoded pixels of a first component and decoded pixels of thesecond component; a candidate generator configured to generatecandidates for an orthogonal transform type to be applied to inverseorthogonal transform processing on transform coefficients of the secondcomponent, the transform coefficients corresponding to predictionresiduals that represent errors between the predicted block of thesecond component and the block of the second component; and an inversetransformer configured to perform the inverse orthogonal transformprocessing on the transform coefficients of the second component byusing an orthogonal transform type selected from among the candidatesgenerated by the candidate generator, wherein the candidate generator isconfigured to generate the candidates for the orthogonal transform type,based on positions of the neighbouring decoded pixels referred to in thecross-component intra prediction.

A program according to a third feature causes a computer to function asthe image encoding device according to the first feature.

A program according to a fourth feature causes a computer to function asthe image decoding device according to the second feature.

Advantageous Effect of Invention

According to the present invention, an image encoding device, an imagedecoding device, and a program can be provided that achieve improvedencoding efficiency when cross-component intra prediction is used.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an image encodingdevice according to an embodiment.

FIG. 2 is a diagram illustrating the normal CCIP mode.

FIG. 3 is a diagram illustrating the CCIP_A mode.

FIG. 4 is a diagram illustrating the CCIP_L mode.

FIG. 5 is a diagram illustrating a configuration of an image decodingdevice according to the embodiment.

FIG. 6 illustrates an example of operation of the image encoding deviceaccording to the embodiment.

FIG. 7 illustrates an example of operation of the image decoding deviceaccording to the embodiment.

FIG. 8 illustrates an example of operation of the image encoding deviceaccording to a modified example 1 of the embodiment.

FIG. 9 illustrates an example of operation of the image decoding deviceaccording to the modified example 1 of the embodiment.

FIG. 10 illustrates an example of operation of the image encoding deviceaccording to a modified example 2 of the embodiment.

FIG. 11 illustrates an example of operation of the image decoding deviceaccording to the modified example 2 of the embodiment.

DESCRIPTION OF EMBODIMENTS

An image encoding device and an image decoding device according to apresent embodiment of the invention are described with reference to theaccompanying drawings. The image encoding device and the image decodingdevice according to the present embodiment encode and decode videos suchas MPEG videos. In the description of the drawings below, the same orsimilar reference signs are used for the same or similar parts.

1. CONFIGURATION OF IMAGE ENCODING DEVICE

FIG. 1 is a diagram illustrating a configuration of the image encodingdevice 1 according to the present embodiment.

As illustrated in FIG. 1 , the image encoding device 1 includes a blockdivider 100, a subtractor 110, a transformer/quantizer 120, an entropyencoder 130, an inverse quantizer/inverse transformer 140, a combiner150, a memory 160, a predictor 170, and a transform controller 180.

The block divider 100 divides an input image given in the form of aframe (or a picture) that constitutes a part of a video into sub-areasin the form of blocks and outputs the resulting blocks to the subtractor110. Each pixel of the input image includes a luminance (Y) and a chroma(C_(b), C_(r)).

The size of the blocks may be 32×32 pixels, 16×16 pixels, 8×8 pixels, or4×4 pixels. The shape of the blocks is not limited to square and may berectangular. A block is the unit of encoding by the image encodingdevice 1 and of decoding by the image decoding device 2.

In the present embodiment, the block divider 100 divides an input imageinto blocks called CU (Coding Unit). One CU includes a luminance blockand chroma blocks. The image encoding device 1 can individually encodethe luminance block and the chroma blocks included in one CU. Similarly,the image decoding device 2 can individually decode the luminance blockand the chroma blocks included in one CU.

The subtractor 110 calculates prediction residuals that represent errorsin units of pixels between a block inputted from the block divider 100and a prediction image (predicted block) obtained by the predictor 170predicting the block. Specifically, the subtractor 110 calculates aprediction residual by subtracting each pixel value in the predictionimage from each pixel value in the block inputted from the block divider100, and outputs the calculated prediction residuals to thetransformer/quantizer 120.

The transformer/quantizer 120 executes an orthogonal transform processand a quantization process on each of blocks. The transformer/quantizer120 includes a transformer 121 and a quantizer 122.

The transformer 121, under control of the transform controller 180,calculates transform coefficients by performing orthogonal transformprocessing on the prediction residuals inputted from the subtractor 110,and outputs the calculated transform coefficients to the quantizer 122.The orthogonal transform processing is, for example, discrete cosinetransformation (DCT), discrete sine transformation (DST), Karhunen-Loevetransformation (KLT), or the like.

In the present embodiment, the transformer 121 performs orthogonaltransform processing in each of a horizontal direction and a verticaldirection by using an orthogonal transform type selected by thetransform controller 180 from among DCT type 2 (DCT-2), DCT type 5(DCT-5), DCT type 8 (DCT-8), and DST type 7 (DST-7). Since theseorthogonal transform types are well known to those skilled in the art, adescription of mathematical expressions and the like of each orthogonaltransform type is omitted.

The quantizer 122 generates quantized transform coefficients byquantizing the transform coefficients inputted from the transformer 121by using a quantization parameter (Qp) and a quantization matrix. Thequantization parameter (Qp) is a parameter that is applied in common toeach transform coefficient in a block, and is a parameter thatdetermines quantization granularity. The quantization matrix is a matrixthat has, as elements, quantization values used when each transformcoefficient is quantized. The quantizer 122 outputs the quantizedtransform coefficients to the entropy encoder 130 and the inversequantizer/inverse transformer 140.

The entropy encoder 130 performs entropy encoding on the transformcoefficients inputted from the quantizer 122, generates encoded data (abit stream) by performing data compression, and outputs the encoded datato an outside of the image encoding device 1. For the entropy encoding,Huffman coding, CABAC (Context-based Adaptive Binary Arithmetic Coding),or the like can be used.

Note that into the entropy encoder 130, control information related toprediction is inputted from the predictor 170, and control informationrelated to transform processing is inputted from the transformcontroller 180. The entropy encoder 130 also performs entropy encodingof such control information.

The inverse quantizer/inverse transformer 140 executes an inversequantization process and an inverse orthogonal transform process on eachof blocks. The inverse quantizer/inverse transformer 140 includes aninverse quantizer 141 and an inverse transformer 142.

The inverse quantizer 141 performs the inverse quantization processcorresponding to the quantization process performed by the quantizer122. More specifically, the inverse quantizer 141 inverse quantizesquantized transform coefficients input from the quantizer 122 using thequantization parameter (Qp) and the quantization matrix to restore thetransform coefficients and outputs the restored transform coefficientsto the inverse transformer 142.

The inverse transformer 142, under control of the transform controller180, performs inverse orthogonal transform processing corresponding tothe orthogonal transform processing performed by the transformer 121.For example, when the transformer 121 performs orthogonal transformprocessing using DCT-2, the inverse transformer 142 performs inverseorthogonal transform processing using DCT-2. The inverse transformer 142restores prediction residuals by performing inverse orthogonal transformprocessing in each of the horizontal direction and the verticaldirection on the transform coefficients inputted from the inversequantizer 141, and outputs the restored prediction residuals to thecombiner 150.

The combiner 150 combines the restored prediction residuals inputtedfrom the inverse transformer 142 with a prediction image inputted fromthe predictor 170, on a pixel-by-pixel basis. The combiner 150reconstructs (decodes) a block by adding individual pixel values of therestored prediction residuals to individual pixel values of theprediction image, and outputs a decoded image on each of decoded blocksto the memory 160. A decoded image is referred to as a reconstructedimage in some cases. A loop filter may be provided between the combiner150 and the memory 160.

The memory 160 stores the decoded image inputted from the combiner 150.The memory 160 stores decoded images in units of frames.

The predictor 170 performs prediction on each of blocks. The predictor170 includes an intra predictor 171, an inter predictor 172 and a switch173.

The intra predictor 171 performs intra prediction on each of blocksobtained by dividing each original image in a unit of a frame. The intrapredictor 171 generates an intra prediction image by referring toneighbouring decoded pixels adjacent to a prediction-target block of adecoded image stored in the memory 160, and outputs the generated intraprediction image to the switcher 173.

The intra predictor 171 selects an optimal intra prediction mode to beapplied to the target block from among a plurality of intra predictionmodes, and performs intra prediction by using the selected intraprediction mode. The intra predictor 171 outputs an index indicating theselected intra prediction mode to the entropy encoder 130. Note thatintra prediction modes include planar prediction, DC prediction,directional prediction, and the like.

The intra predictor 171 can select cross-component intra prediction(CCIP) as an intra prediction mode to be applied to a chroma block of aprediction-target CU. Specifically, the intra predictor 171 includes across-component intra predictor 171 a configured to predict a chromablock of a CU through CCIP by using a decoded image (decoded pixelvalues) of a luminance block of the CU.

Specifically, the cross-component intra predictor 171 a generates apredicted chroma block through cross-component intra prediction in whicha chroma block to be encoded is predicted by referring to, as theneighbouring decoded pixels adjacent to the chroma block, decodedluminance pixels and decoded chroma pixels.

Note that when cross-component intra prediction (CCIP) is applied, theintra predictor 171 first performs intra prediction of a luminance blockof a CU, and a decoded image of the luminance block of the CU is storedin the memory 160.

The cross-component intra predictor 171 a generates the predicted chromablock through cross-component intra prediction (CCIP), and outputs thepredicted chroma block to the subtractor 110 via the switcher 173. Thesubtractor 110 calculates chroma prediction residuals that representerrors between the predicted chroma block and the chroma block inputtedfrom the block divider 100, and outputs the calculated chroma predictionresiduals to the transformer 121. The transformer 121 performsorthogonal transform processing on the chroma prediction residualsinputted from the subtractor 110.

In the present embodiment, CCIP includes the following three modes:normal CCIP, CCIP_A and CCIP_L. FIG. 2 is a diagram illustrating thenormal CCIP mode, FIG. 3 is a diagram illustrating the CCIP_A mode, andFIG. 4 is a diagram illustrating the CCIP_L mode.

As illustrated in FIG. 2 , in a case of the normal CCIP mode, thecross-component intra predictor 171 a calculates a luminance-chromalinear model for a CU by using decoded luminance pixels and decodedchroma pixels adjacent to the top side and the left side of the CU, andpredicts pixel values of a chroma block of the CU by using thecalculated linear model.

In the normal CCIP mode, a chroma signal can be predicted with highaccuracy by using a fact that a relationship between signaldistributions of neighbouring decoded luminance and chroma pixels to thetop side and the left side of a CU can be approximated to a relationshipbetween luminance and chroma signal distributions in the CU. Note that aconcept of neighbouring decoded pixels may include next neighbouringdecoded pixels.

As described in, for example, JVET-G1001-V1 and the like as well as inNon Patent Literatures cited above, the cross-component intra predictor171 a calculates predicted chroma (Cb) pixel values pred_(C)(i, j) of aCU, based on a following expression (1).[Expression 1]pred_(c)(i,j)=α·rec_(L)′(i,j)+β  (1)

where rec_(L)′(i, j) represents downsampled decoded luminance pixelvalues (downsampled reconstructed luma samples) of the CU.

α and β are calculated by the following expressions (2) and (3).

$\begin{matrix}\lbrack {{Expression}2} \rbrack &  \\{\alpha = \frac{{N \cdot {\sum( {{L(n)} \cdot {C(n)}} )}} - {\sum{{L(n)} \cdot {\sum{C(n)}}}}}{{N \cdot {\sum( {{L(n)} \cdot {L(n)}} )}} - {\sum{{L(n)} \cdot {\sum{L(n)}}}}}} & (2)\end{matrix}$ $\begin{matrix}{\beta = \frac{{\sum{C(n)}} - {\alpha \cdot {\sum{L(n)}}}}{N}} & (3)\end{matrix}$

where L(n) represents downsampled top and left neighbouring decodedluminance pixel values (top and left neighbouring reconstructed lumasamples). C(n) represents top and left neighbouring decoded chroma pixelvalues (top and left neighbouring reconstructed chroma samples).

As described above, in the normal CCIP, since neighbouring decodedpixels to the top side and the left side of a CU are always used, acorrect linear model cannot be calculated when a relationship betweenluminance and chroma signal distributions differs between the top sideand left side, and consequently, chroma prediction accuracy is lowered.The CCIP_A mode and the CCIP_L mode are preferred modes in a case wherethe relationship between luminance and chroma signal distributionsdiffers between the top side and left side of a CU.

As illustrated in FIG. 3 , in a case of the CCIP_A mode, thecross-component intra predictor 171 a calculates a linear model by usingneighbouring decoded luminance and chroma pixels only to the top side ofa CU, instead of using neighbouring decoded luminance and chroma pixelsto the top side and the left side of a CU.

When the CCIP_A mode is applied, it is highly probable that predictionaccuracy is high in an area close to the neighbouring decoded pixelsused to calculate the linear model (that is, an upper-end area of achroma block). On the other hand, it is highly probable that predictionaccuracy is low in an area far from the neighbouring decoded pixels usedto calculate the linear model (that is, a lower-end area of the chromablock).

As illustrated in FIG. 4 , in a case of the CCIP_L mode, thecross-component intra predictor 171 a calculates a linear model by usingneighbouring decoded luminance and chroma pixels only to the left sideof a CU, instead of using neighbouring decoded luminance and chromapixels to the top side and the left side of a CU.

When the CCIP_L mode is applied, it is highly probable that predictionaccuracy is high in an area near the neighbouring decoded pixels used tocalculate the linear model (that is, a left-end area of a chroma block),and it is highly probable that prediction accuracy is low in an area farfrom the neighbouring decoded pixels used to calculate the linear model(that is, a right-end area of the chroma block).

When any one of the normal CCIP mode, the CCIP_A mode, and the CCIP_Lmode is selected as an intra prediction mode to be applied to a chromablock, the cross-component intra predictor 171 a outputs an indexindicating the selected mode to the entropy encoder 130 and thetransform controller 180.

The inter predictor 172 calculates a motion vector through a scheme suchas block matching by using, for a reference image, a decoded imagestored in the memory 160, generates an inter prediction image bypredicting a prediction-target block, and outputs the generated interprediction image to the switcher 173. The inter predictor 172 selects anoptimal inter prediction method, from inter prediction using a pluralityof reference images (typically, bi-prediction) and inter predictionusing one reference image (uni-directional prediction), and performsinter prediction by using the selected inter prediction method.

The switcher 173 switches between the intra prediction image inputtedfrom the intra predictor 171 and the inter prediction image inputtedfrom the inter predictor 172, and outputs any one of the predictionimages to the subtractor 110 and the combiner 150.

The transform controller 180 controls the orthogonal transformprocessing performed by the transformer 121 and the inverse orthogonaltransform processing performed by the inverse transformer 142. In thepresent embodiment, when any one of the normal CCIP mode, the CCIP_Amode, and the CCIP_L mode is selected as an intra prediction mode to beapplied to a chroma block, the transform controller 180 controlstransform processing (orthogonal transform processing and inverseorthogonal transform processing) that deals with the chroma block.

The transform controller 180 includes a candidate generator 181 and atype selector 182.

The candidate generator 181 generates candidates for an orthogonaltransform type to be applied to orthogonal transform processing onchroma prediction residuals, based on the index of the intra predictionmode inputted from the cross-component intra predictor 171 a, andoutputs the generated candidates to the type selector 182. For example,the candidates for the orthogonal transform type include at least one ofDCT-2, DCT-5, DCT-8, and DST-7.

Specifically, the candidate generator 181 generates the candidates forthe orthogonal transform type, depending on whether or not positions ofthe neighbouring decoded pixels referred to in cross-component intraprediction (CCIP) are to only any one of a top, a bottom, a left, and aright of a chroma block.

In the present embodiment, the cross-component intra predictor 171 aperforms cross-component intra prediction by using a mode selected fromamong a plurality of modes including a first mode (normal CCIP mode) anda second mode (CCIP_A mode or CCIP_L mode). The first mode is a mode inwhich a chroma block is predicted by referring to neighbouring decodedpixels to both the top side and the left side of the chroma block. Thesecond mode is a mode in which a chroma block is predicted by referringto neighbouring decoded pixels only to the top side or the left side ofthe chroma block.

When the first mode is selected, the candidate generator 181 generatesfirst candidates that include one or more orthogonal transform types, asthe candidates for the orthogonal transform type. When the second modeis selected, the candidate generator 181 generates second candidatesthat are different from the first candidates, as the candidates for theorthogonal transform type. In other words, the candidate generator 181generates the candidates for the orthogonal transform type that differbetween the first mode (normal CCIP mode) and the second mode (CCIP_Amode or CCIP_L mode).

Here, the first candidates and the second candidates each includehorizontal-direction candidates that are one or more candidates for anorthogonal transform type to be applied to transform processing in thehorizontal direction, and vertical-direction candidates that are one ormore candidates for an orthogonal transform type to be applied totransform processing in the vertical direction. In the first candidates,the horizontal-direction candidates and the vertical-directioncandidates may be identical. On the other hand, in the secondcandidates, the horizontal-direction candidates and thevertical-direction candidates are different from each other.

The type selector 182 selects, from among the candidates inputted fromthe candidate generator 181, orthogonal transform types to be applied toorthogonal transform processing on the chroma prediction residuals, andoutputs an index (flag) indicating the selected orthogonal transformtypes to the transformer 121, the inverse transformer 142, and theentropy encoder 130. For example, the type selector 182 may calculate anRD (Rate Distortion) cost for each candidate inputted from the candidategenerator 181, and may select an orthogonal transform type or types fromamong the candidates, based on the calculated RD costs.

Specifically, the type selector 182 selects an orthogonal transform typeto be applied to transform processing in the horizontal direction fromamong the horizontal-direction candidates inputted from the candidategenerator 181, and outputs an index (flag) indicating the selectedorthogonal transform type in the horizontal direction to the transformer121, the inverse transformer 142, and the entropy encoder 130.

However, when the horizontal-direction candidates inputted from thecandidate generator 181 include only one orthogonal transform type, theindex (flag) indicating the orthogonal transform type in the horizontaldirection need not be outputted to the entropy encoder 130 because anorthogonal transform type to be applied to transform processing in thehorizontal direction is uniquely determined.

Moreover, the type selector 182 selects an orthogonal transform type tobe applied to transform processing in the vertical direction from amongthe vertical-direction candidates inputted from the candidate generator181, and outputs an index (flag) indicating the selected orthogonaltransform type in the vertical direction to the transformer 121, theinverse transformer 142, and the entropy encoder 130.

However, when the vertical-direction candidates inputted from thecandidate generator 181 include only one orthogonal transform type, theindex (flag) indicating the orthogonal transform type in the verticaldirection need not be outputted to the entropy encoder 130 because anorthogonal transform type to be applied to transform processing in thevertical direction is uniquely determined.

2. CONFIGURATION OF IMAGE DECODING DEVICE

FIG. 5 illustrates the configuration of an image decoding device 2according to the present embodiment.

As illustrated in FIG. 5 , the image decoding device 2 includes anentropy code decoder 200, an inverse quantizer/inverse transformer 210,a combiner 220, a memory 230, a predictor 240, and a transformcontroller 250.

The entropy code decoder 200 decodes encoded data generated by the imageencoding device 1, and outputs quantized transform coefficients to theinverse quantizer/inverse transformer 210. Moreover, the entropy codedecoder 200 acquires control information related to prediction (intraprediction and inter prediction), and outputs the acquired controlinformation to the predictor 240.

In the present embodiment, the entropy code decoder 200 acquires anindex of an intra prediction mode (specifically, a mode of CCIP) and anindex of an orthogonal transform type in each of the horizontaldirection and the vertical direction, outputs the index of the intraprediction mode to an intra predictor 241 and a type selector 252, andoutputs the index of the orthogonal transform types to the type selector252.

The inverse quantizer/inverse transformer 210 performs inversequantization processing and inverse orthogonal transform processing oneach of blocks. The inverse quantizer/inverse transformer 210 includesan inverse quantizer 211 and an inverse transformer 212.

The inverse quantizer 211 performs the inverse quantization processcorresponding to the quantization process performed by the quantizer 122of the image encoding device 1. The inverse quantizer 211 inversequantizes quantized transform coefficients input from the entropy codedecoder 200 using the quantization parameter (Qp) and the quantizationmatrix to restore the transform coefficients and outputs the restoredtransform coefficients to the inverse transformer 212.

The inverse transformer 212, under control of the transform controller250, performs inverse orthogonal transform processing corresponding toorthogonal transform processing performed by the transformer 121 of theimage encoding device 1. The inverse transformer 212 restores predictionresiduals by performing inverse orthogonal transform processing in eachof the horizontal direction and the vertical direction on the transformcoefficients inputted from the inverse quantizer 211, and outputs therestored prediction residuals to the combiner 220.

The combiner 220 reconstructs (decodes) an original block by combiningthe prediction residuals inputted from the inverse transformer 212 and aprediction image (predicted block) inputted from predictor 240 on apixel-by-pixel basis, and outputs a decoded image on each of blocks tothe memory 230. Note that a loop filter may be provided between thecombiner 220 and the memory 230.

The memory 230 stores the decoded image inputted from the combiner 220.The memory 230 stores decoded images in units of frames. The memory 230outputs the decoded images in units of frames to an outside of the imagedecoding device 2.

The predictor 240 performs prediction on each of blocks. The predictor240 includes the intra predictor 241, an inter predictor 242, and aswitcher 243.

The intra predictor 241 generates an intra prediction image by referringto neighbouring decoded pixels adjacent to a prediction-target block ofa decoded image stored in the memory 230, and outputs the generatedintra prediction image to the switcher 243. Moreover, the intrapredictor 241 selects an intra prediction mode from among a plurality ofintra prediction modes, based on the index of the intra prediction modeinputted from the entropy code decoder 200, and performs intraprediction by using the selected intra prediction mode.

The intra predictor 241 includes a cross-component intra predictor 241 aconfigured to predict a chroma block of a CU through cross-componentintra prediction (CCIP) by using a decoded image (decoded pixel values)of a luminance block of the CU. Specifically, the cross-component intrapredictor 241 a generates a predicted chroma block throughcross-component intra prediction in which a chroma block to be decodedis predicted by referring to, as the neighbouring decoded pixelsadjacent to the chroma block, decoded luminance pixels and decodedchroma pixels.

Note that when cross-component intra prediction (CCIP) is applied, theintra predictor 241 first performs intra prediction of a luminance blockof a CU, and a decoded image of the luminance block of the CU is storedin the memory 230.

The cross-component intra predictor 241 a generates the predicted chromablock through cross-component intra prediction (CCIP), and outputs thepredicted chroma block to the combiner 220 via the switcher 243. Thecombiner 220 reconstructs (decodes) an original chroma block bycombining the chroma prediction residuals inputted from the inversetransformer 212 and the predicted chroma block inputted from thecross-component intra predictor 241 a on a pixel-by-pixel basis, andoutputs a decoded image on each of blocks to the memory 230.

In the present embodiment, CCIP includes three modes, namely, normalCCIP, CCIP_A, and CCIP_L. When any one of the normal CCIP mode, theCCIP_A mode, and the CCIP_L mode is selected as an intra prediction modeto be applied to a chroma block, the cross-component intra predictor 241a outputs an index indicating the selected mode to the transformcontroller 250.

The inter predictor 242 performs inter prediction that predicts aprediction-target block by using, for a reference image, a decoded imagestored in the memory 230. The inter predictor 242 generates an interprediction image by performing inter prediction in accordance withcontrol information (motion vector information and the like) inputtedfrom the entropy code decoder 200, and outputs the generated interprediction image to the switcher 243.

The switcher 243 switches between the intra prediction image inputtedfrom the intra predictor 241 and the inter prediction image inputtedfrom the inter predictor 242, and outputs any one of the predictionimages to the combiner 220.

The transform controller 250 controls inverse orthogonal transformprocessing performed by the inverse transformer 212. In the presentembodiment, when any one of the normal CCIP mode, the CCIP_A mode, andthe CCIP_L mode is selected as an intra prediction mode to be applied toa chroma block, the transform controller 250 controls transformprocessing (inverse orthogonal transform processing) that deals with thechroma block. The transform controller 250 includes a candidategenerator 251 and the type selector 252.

The candidate generator 251 generates candidates for an orthogonaltransform type to be applied to inverse orthogonal transform processingperformed by the inverse transformer 212, based on the index of theintra prediction mode inputted from the cross-component intra predictor241 a, and outputs the generated candidates to the type selector 252.For example, the candidates for the orthogonal transform type include atleast one of DCT-2, DCT-5, DCT-8, and DST-7.

Specifically, the candidate generator 251 generate the candidates forthe orthogonal transform type, depending on whether or not positions ofthe neighbouring decoded pixels referred to in cross-component intraprediction (CCIP) are to only any one of the top, the bottom, the left,and the right of a chroma block.

In the present embodiment, the cross-component intra predictor 241 aperforms cross-component intra prediction by using a mode selected fromamong a plurality of modes including the first mode (normal CCIP mode)and the second mode (CCIP_A mode or CCIP_L mode). The first mode is amode in which a chroma block is predicted by referring to neighbouringdecoded pixels to both the top side and the left side of the chromablock. The second mode is a mode in which a chroma block is predicted byreferring to neighbouring decoded pixels only to the top side or theleft side of the chroma block.

When the first mode is selected, the candidate generator 251 generatesfirst candidates including one or more orthogonal transform types, asthe candidates for the orthogonal transform type. When the second modeis selected, the candidate generator 251 generates second candidatesthat are different from the first candidates, as the candidates for theorthogonal transform type. In other words, the candidate generator 251generates the candidates for the orthogonal transform type that differbetween the first mode (normal CCIP mode) and the second mode (CCIP_Amode or CCIP_L mode).

Here, the first candidates and the second candidates each includehorizontal-direction candidates that are one or more candidates for anorthogonal transform type to be applied to transform processing in thehorizontal direction, and vertical-direction candidates that are one ormore candidates for an orthogonal transform type to be applied totransform processing in the vertical direction. In the first candidates,the horizontal-direction candidates and the vertical-directioncandidates may be identical. On the other hand, in the secondcandidates, the horizontal-direction candidates and thevertical-direction candidates are different from each other.

The type selector 252 selects orthogonal transform types to be appliedto inverse orthogonal transform processing (in the horizontal direction,the vertical direction) from among the candidates inputted from thecandidate generator 251, in accordance with the index of the orthogonaltransform types (in the horizontal direction, the vertical direction)inputted from the entropy code decoder 200. Specifically, the index ofthe orthogonal transform types inputted from the entropy code decoder200 specifies any of the candidates generated by the candidate generator251. The type selector 252 outputs an index (flag) indicating theselected orthogonal transform types to the inverse transformer 212.

3. OPERATION EXAMPLES

Hereinafter, examples of operation of the image encoding device 1 andthe image decoding device 2 according to the present embodiment will bedescribed.

<3.1. Example of Operation of Image Encoding Device>

FIG. 6 illustrates an example of operation of the image encoding device1 according to the present embodiment.

As illustrated in FIG. 6 , in step S101, the intra predictor 171 selectsan intra prediction mode for a chroma block. Here, a description will becontinued, assuming that any one of the normal CCIP mode, the CCIP_Amode, and the CCIP_L mode is selected. The cross-component intrapredictor 171 a performs cross-component intra prediction according tothe selected mode, and generates a predicted chroma block. Thesubtractor 110 calculates chroma prediction residuals that representerrors between the original chroma block and the predicted chroma block.

(Normal CCIP Mode)

When the normal CCIP mode is selected (step S102: YES), in step S103,the candidate generator 181 generates candidates including DCT-2, DCT-8,and DST-7, as horizontal-direction candidates. The candidate generator181 generates candidates including DCT-2, DCT-8, and DST-7, asvertical-direction candidates.

In step S107, the type selector 182 determines whether to apply DCT-2 inthe vertical direction and the horizontal direction, or to selectivelyapply DCT-8 or DST-7 in each of the vertical direction and thehorizontal direction.

In step S108, the transformer 121 generates transform coefficients byperforming orthogonal transform processing (in the horizontal direction,the vertical direction) on the chroma prediction residuals, by using theorthogonal transform type or types selected by the type selector 182.

In step S109, the entropy encoder 130 encodes the transform coefficientsthat are quantized and an index indicating the normal CCIP mode.

Moreover, in step S109, the entropy encoder 130 encodes an indexindicating the selected orthogonal transform type or types. For example,the index includes a first flag and a second flag. Specifically, whenDCT-2 is applied in the horizontal direction and the vertical direction,the first flag is set to “1”, and then encoded and transmitted. WhenDCT-8 or DST-7 is selectively applied in each of the vertical directionand the horizontal direction, the first flag is set to “0”, and thenencoded and transmitted, and thereafter the second flag indicating whichone of DCT-8 and DST-7 is applied as the orthogonal transform type inthe horizontal direction is encoded and outputted in a bit stream.Similarly, the second flag indicating which one of DCT-8 and DST-7 isapplied as the orthogonal transform type in the vertical direction isencoded and outputted in a bit stream.

(CCIP_L Mode)

When the CCIP_L mode is selected (step S104: YES), in step S105, thecandidate generator 181 generates a candidate including only DST-7, as ahorizontal-direction candidate. The candidate generator 181 generatescandidates including DCT-8 and DST-7, as vertical-direction candidates.As described above, when the CCIP_L mode is applied to a chroma block,energy in a left-side area of prediction residuals tends to be smaller,and energy in a right-side area of the prediction residuals tends to belarger. Accordingly, the candidate generator 181 is configured togenerate a candidate including only DST-7, as a horizontal-directioncandidate.

In step S107, the type selector 182 selects any one of DCT-8 and DST-7for the vertical direction. As for the horizontal direction, since DST-7is the only candidate, the type selector 182 selects DST-7 for thehorizontal direction.

In step S108, the transformer 121 generates transform coefficients byperforming orthogonal transform processing (in the horizontal direction,the vertical direction) on the chroma prediction residuals, by using theorthogonal transform types selected by the type selector 182.

In step S109, the entropy encoder 130 encodes the transform coefficientsthat are quantized and an index indicating the CCIP_L mode.

Moreover, in step S109, the entropy encoder 130 encodes an indexindicating the selected orthogonal transform types. However, since thereis only one orthogonal transform candidate in the horizontal direction,the entropy encoder 130 does not entropy-encode a flag indicating theorthogonal transform type to be applied in the horizontal direction, andentropy-encodes flags (the first flag and the second flag describedabove) indicating the orthogonal transform type to be applied in thevertical direction. Since entropy encoding of the flag indicating theorthogonal transform type to be applied in the horizontal direction isnot required, the number of flags transmitted to the image decodingdevice 2 can be reduced.

(CCIP_A mode)

When the CCIP_A mode is selected (step S104: NO), in step S106, thecandidate generator 181 generates a candidate including only DST-7, as avertical-direction candidate. The candidate generator 181 generatescandidates including DCT-8 and DST-7, as horizontal-directioncandidates. As described above, when the CCIP_A mode is applied to achroma block, energy in a top-side area of prediction residuals tends tobe smaller, and energy in a bottom-side area of the prediction residualstends to be lager. Accordingly, the candidate generator 181 isconfigured to generate a candidate including only DST-7, as avertical-direction candidate.

In step S107, the type selector 182 selects any one of DCT-8 and DST-7for the horizontal direction. As for the vertical direction, since DST-7is the only candidate, the type selector 182 selects DST-7 for thehorizontal direction.

In step S108, the transformer 121 generates transform coefficients byperforming orthogonal transform processing (in the horizontal direction,the vertical direction) on the chroma prediction residuals, by using theorthogonal transform types selected by the type selector 182.

In step S109, the entropy encoder 130 encodes the transform coefficientsthat are quantized and an index indicating the CCIP_A mode.

Moreover, in step S109, the entropy encoder 130 encodes an indexindicating the selected orthogonal transform types. However, since thereis only one orthogonal transform candidate in the vertical direction,the entropy encoder 130 does not entropy-encode a flag indicating theorthogonal transform type to be applied in the vertical direction, andentropy-encodes flags (the first flag and the second flag describedabove) indicating the orthogonal transform type to be applied in thehorizontal direction. Since entropy encoding of the flag indicating theorthogonal transform type to be applied in the vertical direction is notrequired, the number of flags transmitted to the image decoding device 2can be reduced.

<3.2. Example of Operation of Image Decoding Device>

FIG. 7 illustrates an example of operation of the image decoding device2 according to the present embodiment.

As illustrated in FIG. 7 , in step S201, the entropy code decoder 200decodes encoded data generated by the image encoding device 1, andoutputs quantized transform coefficients to the inversequantizer/inverse transformer 210. Moreover, the entropy code decoder200 acquires an index of an intra prediction mode and an index of anorthogonal transform type of types (in the horizontal direction, thevertical direction). Here, a description will be continued, assumingthat the index of the intra prediction mode indicates any one of thenormal CCIP mode, the CCIP_A mode, and the CCIP_L mode. Thecross-component intra predictor 241 a generates a predicted chroma blockby performing cross-component intra prediction according to the modeindicated by the index.

(Normal CCIP Mode)

In a case of the normal CCIP mode (step S202: YES), in step S203, thecandidate generator 251 generates candidates including DCT-2, DCT-8, andDST-7, as horizontal-direction candidates. The candidate generator 251generates candidates including DCT-2, DCT-8, and DST-7, asvertical-direction candidates.

In step S207, the type selector 252 selects, from among the candidates(in the horizontal direction, the vertical direction) generated by thecandidate generator 251, an orthogonal transform type (in the horizontaldirection, the vertical direction each) to be applied to inverseorthogonal transform processing on the transform coefficients, based onthe index of the orthogonal transform type (in the horizontal direction,the vertical direction each) inputted from the entropy code decoder 200.

In step S208, the inverse transformer 212 restores chroma predictionresiduals by performing inverse orthogonal transform processing (in thehorizontal direction, the vertical direction) on the transformcoefficients, by using the orthogonal transform type selected by thetype selector 252. Thereafter, the combiner 220 restores (decodes) anoriginal chroma block by synthesizing the restored chroma predictionresiduals and the predicted chroma block generated by thecross-component intra predictor 241 a.

(CCIP_L mode)

In a case of the CCIP_L mode (step S204: YES), in step S205, thecandidate generator 251 generates a candidate including only DST-7, as ahorizontal-direction candidate. The candidate generator 251 generatescandidates including DCT-8 and DST-7, as vertical-direction candidates.

In step S207, the type selector 252 selects any one of DCT-8 and DST-7for the vertical direction, based on the index of the orthogonaltransform type (in the vertical direction) inputted from the entropycode decoder 200. As for the horizontal direction, since DST-7 is onlythe candidate, the type selector 252 selects DST-7 for the horizontaldirection.

In step S208, the inverse transformer 212 restores chroma predictionresiduals by performing inverse orthogonal transform processing (in thehorizontal direction, the vertical direction) on the transformcoefficients, by using the orthogonal transform types selected by thetype selector 252. Thereafter, the combiner 220 restores (decodes) anoriginal chroma block by synthesizing the restored chroma predictionresiduals and the predicted chroma block generated by thecross-component intra predictor 241 a.

(CCIP_A mode)

In a case of the CCIP_A mode (step S204: NO), in step S206, thecandidate generator 251 generates a candidate including only DST-7, as avertical-direction candidate. The candidate generator 251 generatescandidates including DCT-8 and DST-7, as horizontal-directioncandidates.

In step S207, the type selector 252 selects any one of DCT-8 and DST-7for the horizontal direction, based on the index of the orthogonaltransform type (in the horizontal direction) inputted from the entropycode decoder 200. As for the vertical direction, since DST-7 is the onlycandidate, the type selector 252 selects DST-7 for the horizontaldirection.

In step S208, the inverse transformer 212 restores chroma predictionresiduals by performing inverse orthogonal transform processing (in thehorizontal direction, the vertical direction) on the transformcoefficients, by using the orthogonal transform types selected by thetype selector 252. Thereafter, the combiner 220 restores (decodes) anoriginal chroma block by synthesizing the restored chroma predictionresiduals and the predicted chroma block generated by thecross-component intra predictor 241 a.

4. SUMMARY OF EMBODIMENT

As described above, according to the image encoding device and the imagedecoding device 2 in the present embodiment, even when positions ofneighbouring decoded pixels referred to in cross-component intraprediction are to only any one of the top, the bottom, the left, and theright of a chroma block (that is, in a case of the CCIP_A mode or theCCIP_L mode), transform processing (orthogonal transform processing andinverse orthogonal transform processing) can be performed by using anorthogonal transform type or types suitable to an energy distributiontrend of prediction residuals. Accordingly, encoding efficiency can beimproved.

According to the image encoding device 1 and the image decoding device 2in the present embodiment, by generating candidates for the orthogonaltransform type that differ between the first mode (normal CCIP mode) andthe second mode (CCIP_A mode or CCIP_L mode), transform processing(orthogonal transform processing and inverse orthogonal transformprocessing) can be performed by using an orthogonal transform type ortypes suitable to an energy distribution trend of prediction residualsin the second mode. Accordingly, encoding efficiency can be improved.

According to the image encoding device 1 and the image decoding device 2in the present embodiment, when the second mode is selected, bygenerating candidates for the orthogonal transform type that differbetween transform processing in the horizontal direction and transformprocessing in the vertical direction, transform processing can beperformed by using an orthogonal transform type suitable to an energydistribution trend of prediction residuals in each of the horizontaldirection and the vertical direction. Accordingly, encoding efficiencycan be improved.

5. MODIFIED EXAMPLE 1

In the above-described operation examples according to the embodiment,an example is described in which when only one candidate is generated asa candidate for the orthogonal transform type in the horizontaldirection or the vertical direction, the candidate is generated by usingDST-7 as an orthogonal transform candidate. However, any transformationwill do if the transformation, similarly to DST-7, has asymmetricimpulse responses and is an orthogonal transformtion generating smallercoefficients for end points on one side and larger coefficients for endpoints on another side, and a type of such a transform is not limited toDST-7. For example, DCT-5 that has characteristics similar to DST-7 maybe used in place of DST-7.

Alternatively, DCT-5 and DST-7 may be selectively applied. FIG. 8illustrates an example of operation of the image encoding device 1according to the present modified example. FIG. 9 illustrates an exampleof operation of the image decoding device 2 according to the presentmodified example. Here, differences from the above-described operationexamples according to the embodiment will be described.

As illustrated in FIG. 8 , in a case of CCIP_L (step S104: YES), in stepS105 a, the candidate generator 181 generates candidates including DCT-5and DST-7, as horizontal-direction candidates. The candidate generator181 generates candidates including DCT-8 and DST-7, asvertical-direction candidates. In such a case, in step S107, the typeselector 182 selects any one of DCT-5 and DST-7 for the horizontaldirection, and selects any one of DCT-8 and DST-7 for the verticaldirection.

In a case of CCIP_A (step S104: NO), in step S106 a, the candidategenerator 181 generates candidates including DCT-5 and DST-7, asvertical-direction candidates. The candidate generator 181 generatescandidates including DCT-8 and DST-7, as horizontal-directioncandidates.

In step S107, the type selector 182 selects any one of DCT-5 and DST-7for the vertical direction, and selects any one of DCT-8 and DST-7 forthe horizontal direction.

As illustrated in FIG. 9 , in a case of the CCIP_L mode (step S204:YES), in step S205 a, the candidate generator 251 generates candidatesincluding DCT-5 and DST-7, as horizontal-direction candidates. Thecandidate generator 251 generates candidates including DCT-8 and DST-7,as vertical-direction candidates.

In step S207, the type selector 252 selects any one of DCT-5 and DST-7for the horizontal direction, based on the index of the orthogonaltransform type (in the horizontal direction) inputted from the entropycode decoder 200, and selects any one of DCT-8 and DST-7 for thevertical direction, based on the index of the orthogonal transform type(in the vertical direction) inputted from the entropy code decoder 200.

In a case of the CCIP_A mode (step S204: NO), in step S206 a, thecandidate generator 251 generates candidates including DCT-5 and DST-7,as vertical-direction candidates. The candidate generator 251 generatescandidates including DCT-8 and DST-7, as horizontal-directioncandidates.

In step S207, the type selector 252 selects any one of DCT-5 and DST-7for the vertical direction, based on the index of the orthogonaltransform type (in the vertical direction) inputted from the entropycode decoder 200, and selects any one of DCT-8 and DST-7 for thehorizontal direction, based on the index of the orthogonal transformtype (in the horizontal direction) inputted from the entropy codedecoder 200.

Note that although the present modified example describes an example inwhich candidates are generated by using DST-7 and DCT-5 as orthogonaltransform candidates, types of a transformation are not limited to DST-7and DCT-5 if the transformation has characteristics similar to DST-7 orDCT-5 and has asymmetric impulse responses.

6. MODIFIED EXAMPLE 2

In the above-described embodiment and the modified example 1, thenumbers of orthogonal transform candidates generated when the CCIP_Lmode is applied and when the CCIP_A mode is applied are made smallerthan the number of orthogonal transform candidates in the normal CCIPmode, whereby a bit length of a flag transmitted from the image encodingdevice 1 to the image decoding device 2 can be reduced. However, aconfiguration as described below may also be made.

Specifically, in the present modified example, when the CCIP_A mode isapplied, application of DCT-8 in the vertical direction is prohibited sothat DCT-8 is not included in vertical-direction candidates. When theCCIP_L mode is applied, application of DCT-8 in the horizontal directionis prohibited so that DCT-8 is not included in horizontal-directioncandidates. Other orthogonal transform candidates are similar to theorthogonal transform candidates in the normal CCIP mode.

Next, FIG. 10 illustrates an example of operation of the image encodingdevice 1 according to the present modified example. FIG. 11 illustratesan example of operation of the image decoding device 2 according to thepresent modified example. A description will be given of differencesfrom the above-described operation examples according to the embodiment.

As illustrated in FIG. 10 , when the CCIP_L mode is applied, thecandidate generator 181 generates candidates including DCT-2 and DST-7,as horizontal-direction candidates, and generates candidates includingDCT-2, DCT-8, and DST-7, as vertical-direction candidates (step S105 b).

In step S107, the type selector 182 selects any one of DCT-2 and DST-7for the horizontal direction, and selects any one of DCT-2, DCT-8, andDST-7 for the vertical direction.

When the CCIP_A mode is applied, the candidate generator 181 generatescandidates including DCT-2 and DST-7, as vertical-direction candidates,and generates candidates including DCT-2, DCT-8, and DST-7, ashorizontal-direction candidates (step S106 b).

In step S107, the type selector 182 selects any one of DCT-2 and DST-7for the vertical direction, and selects any one of DCT-2, DCT-8, andDST-7 for the horizontal direction.

As illustrated in FIG. 11 , when the CCIP_L mode is applied, thecandidate generator 251 generates candidates including DCT-2 and DST-7,as horizontal-direction candidates, and generates candidates includingDCT-2, DCT-8, and DST-7, as vertical-direction candidates (step S205 b).

In step S207, the type selector 252 selects any one of DCT-2 and DST-7for the horizontal direction, based on the index of the orthogonaltransform type (in the horizontal direction) inputted from the entropycode decoder 200. The type selector 252 selects any one of DCT-2, DCT-8,and DST-7 for the vertical direction, based on the index of theorthogonal transform type (in the vertical direction) inputted from theentropy code decoder 200. When the CCIP_A mode is applied, the candidategenerator 251 generates candidates including DCT-2 and DST-7, asvertical-direction candidates, and generates candidates including DCT-2,DCT-8, and DST-7, as horizontal-direction candidates.

In step S207, the type selector 252 selects any one of DCT-2 and DST-7for the vertical direction, based on the index of the orthogonaltransform type (in the vertical direction) inputted from the entropycode decoder 200, and selects any one of DCT-2, DCT-8, and DST-7 for thehorizontal direction, based on the index of the orthogonal transformtype (in the horizontal direction) inputted from the entropy codedecoder 200.

7. OTHER EMBODIMENTS

In the above-described embodiment, an example is described in which forthe normal CCIP mode, a CCIP mode is used in which a chroma block ispredicted by referring to neighbouring decoded luminance and chromapixels to the top side and the left side of the chroma block. However,instead of such a mode, a mode may be used in which a chroma block ispredicted by referring to neighbouring decoded luminance and chromapixels to a bottom side and a right side of the chroma block.

In the above-described embodiment, an example is described in which theCCIP_A mode is used in which a chroma block is predicted by referring toneighbouring decoded luminance and chroma pixels only to the top side ofthe chroma block. However, instead of such a mode, a mode may be used inwhich a chroma block is predicted by referring to neighbouring decodedluminance and chroma pixels only to the bottom side of the chroma block.

In the above-described embodiment, an example is described in which theCCIP_L mode is used in which a chroma block is predicted by referring toneighbouring decoded luminance and chroma pixels only to the left sideof the chroma block. However, instead of such a mode, a mode may be usedin which a chroma block is predicted by referring to neighbouringdecoded luminance and chroma pixels only to the right side of the chromablock.

In the above-described embodiment, orthogonal transform candidates mayinclude a transform skip in which a transformation is not applied to aresidual signal. The transform skip is to scale a residual signal intotransform coefficients, without applying a transformation such as DCT orDST.

In the embodiment described above, by way of example, a video signalconsists of luminance-chroma components (YCbCr). A video signal may,however, consist of three primary color components (RGB).

A program may be provided to cause a computer to execute the operationsof the image encoding device 1 and a program may be provided to cause acomputer to execute the operations of the image decoding device 2. Theprogram may be stored in a computer-readable medium. The program can beinstalled on a computer from a computer-readable medium having theprogram stored thereon. The computer-readable medium having the programstored thereon may be a non-transitory recording medium. Thenon-transitory recording medium may include, but is not limited to, aCD-ROM and a DVD-ROM. The image encoding device 1 may be embodied as asemiconductor integrated circuit (chipset, SoC, etc.) by integrating thecircuits that execute the respective operations of the image encodingdevice 1. Similarly, the image decoding device 2 may be embodied as asemiconductor integrated circuit by integrating the circuits thatexecute the respective operations of the image decoding device 2.

The embodiments have been described in detail above with reference tothe drawings. Specific configurations are not limited to theabove-described configurations, and various design changes, and the likeare possible within the scope not deviating from the gist.

This application claims the priority of Japanese Patent Application No.2018-152988 (filed on Aug. 15, 2018) the contents of which are allincorporated herein by reference.

The invention claimed is:
 1. An image encoding device comprising: across-component predictor configured to generate a predicted block of asecond component through cross-component prediction in which a block ofthe second component is predicted by referring to decoded pixels of afirst component and decoded pixels of the second component; a candidategenerator configured to generate candidates for a transform type to beapplied to transform processing on prediction residuals that representerrors between the predicted block of the second component and the blockof the second component; and a transformer configured to perform thetransform processing on the prediction residuals by using a transformtype selected from among the candidates generated by the candidategenerator, wherein the candidate generator is configured to generate thecandidates for the transform type, based on a mode indicating a type ofa process of the cross-component prediction.
 2. The image encodingdevice according to claim 1, wherein the first component is a luminancecomponent, and the second component is a chroma component.
 3. The imageencoding device according to claim 1, wherein the candidate generator isconfigured to generate the candidates for the transform type, dependingon whether or not positions of neighbouring decoded pixels referred toin the cross-component prediction are to only any one of a top, abottom, a left, and a right of the block of the second component.
 4. Theimage encoding device according to claim 1, wherein the cross-componentpredictor is configured to perform the cross-component prediction byusing a mode selected from among a plurality of modes including a firstmode and a second mode, the first mode is a mode in which the block ofthe second component is predicted by referring to the neighbouringdecoded pixels to both a top side and a left side of the block of thesecond component, the second mode is a mode in which the block of thesecond component is predicted by referring to the neighbouring decodedpixels only to the top side or the left side of the block of the secondcomponent, and the candidate generator is configured to generate firstcandidates including one or more transform types, as the candidates forthe transform type when the first mode is selected, and generate secondcandidates that are different from the first candidates, as thecandidates for the transform type when the second mode is selected. 5.The image encoding device according to claim 4, wherein the secondcandidates include horizontal-direction candidates that are one or morecandidates for a transform type to be applied to the transformprocessing in a horizontal direction, and vertical-direction candidatesthat are one or more candidates for a transform type to be applied tothe transform processing in a vertical direction, and thehorizontal-direction candidates and the vertical-direction candidatesare at least partially different from each other.
 6. An image decodingdevice comprising: a cross-component predictor configured to generate apredicted block of a second component through cross-component predictionin which a block of the second component is predicted by referring todecoded pixels of a first component and decoded pixels of the secondcomponent; a candidate generator configured to generate candidates for atransform type to be applied to inverse transform processing ontransform coefficients of the second component, the transformcoefficients corresponding to prediction residuals that represent errorsbetween the predicted block of the second component and the block of thesecond component; and an inverse transformer configured to perform theinverse transform processing on the transform coefficients of the secondcomponent by using a transform type selected from among the candidatesgenerated by the candidate generator, wherein the candidate generator isconfigured to generate the candidates for the transform type, based on amode indicating a type of a process of the cross-component prediction.7. The image decoding device according to claim 6, wherein the firstcomponent is a luminance component, and the second component is a chromacomponent.
 8. The image decoding device according to claim 6, whereinthe candidate generator is configured to generate the candidates for thetransform type, depending on whether or not positions of neighbouringdecoded pixels referred to in the cross-component prediction are to onlyany one of a top, a bottom, a left, and a right of the block of thesecond component.
 9. The image decoding device according to claim 6,wherein the cross-component predictor is configured to perform thecross-component prediction by using a mode selected from among aplurality of modes including a first mode and a second mode, the firstmode is a mode in which the block of the second component is predictedby referring to the neighbouring decoded pixels to both a top side and aleft side of the block of the second component, the second mode is amode in which the block of the second component is predicted byreferring to the neighbouring decoded pixels only to the top side or theleft side of the block of the second component, and the candidategenerator is configured to generate first candidates including one ormore transform types, as the candidates for the transform type when thefirst mode is selected, and generate second candidates that aredifferent from the first candidates, as the candidates for the transformtype when the second mode is selected.
 10. The image decoding deviceaccording to claim 9, wherein the second candidates includehorizontal-direction candidates that are one or more candidates for atransform type to be applied to the inverse transform processing in ahorizontal direction, and vertical-direction candidates that are one ormore candidates for a transform type to be applied to the inversetransform processing in a vertical direction, and thehorizontal-direction candidates and the vertical-direction candidatesare at least partially different from each other.
 11. An image decodingmethod performed at an image decoding device, comprising: generating apredicted block of a second component through cross-component predictionin which a block of the second component is predicted by referring todecoded pixels of a first component and decoded pixels of the secondcomponent; generating candidates for a transform type to be applied toinverse transform processing on transform coefficients of the secondcomponent, the transform coefficients corresponding to predictionresiduals that represent errors between the predicted block of thesecond component and the block of the second component; and performingthe inverse transform processing on the transform coefficients of thesecond component by using a transform type selected from among thegenerated candidates, wherein the generating the candidates comprisesgenerating the candidates for the transform type, based on a modeindicating a type of a process of the cross-component prediction.